1. Field of the Invention
The invention is in the field of synchronizing the timing and frequency of a multimode terminal with at least two different base stations, and more specifically relates to a multimode communication terminal such as, for example, a dual-mode GSM-UMTS terminal (Global System for Mobile Communications and Universal Mobile Telecommunications System respectively), comprising a first modulation/demodulation circuit adapted to communicate with a first radio access network and at least a second modulation/demodulation circuit adapted to communicate with a second radio access network different from the first radio network.
The invention also relates to a method of evaluating transmission quality of a multimode terminal over at least two different radio access networks.
2. Description of the Related Art
Currently GSM is the mobile telephone system of reference in Europe and in the world. In order to ensure its evolution towards the third generation, international standardization organizations have proposed UMTS, technical specifications of which are developed under a partnership called 3GPP. One thing desired in the process of standardizing UMTS was to ensure its interoperability with GSM. Also, a specific classification describing four types of dual-mode terminals GSM/UMTS, Type 1, Type 2, Type 3, and Type 4 was established.
For terminals of Type 1, when a terminal is located in a radio GSM network or in a radio UTRA (for Universal Terrestrial Radio Access) network, the “inactive” radio part of the terminal (UTRA or GSM) does not perform any measurement of reception quality, with an implication that transition from one mode to the other cannot be made without intervention by the user.
For terminals of Type 2, when a mobile terminal is connected to a radio GSM network or to a radio UMTS network, the inactive radio part of the terminal (UTRA or GSM) may perform measurements in order to switch automatically to the network that provides the better reception quality.
Terminals of Type 3 are different from those of Type 2 in that they further provide the possibility of receiving information simultaneously in the two modes. On the other hand, simultaneous transmission in the two modes is not possible. As with terminals of Type 2, transition from one radio network to another takes place automatically.
For terminals of Type 4, transition from one radio network to another takes place automatically, and information may be received and transmitted simultaneously over the two networks.
Among the four types of terminals in this classification, it is the dual-mode terminals of Type 2 that are most common. These terminals allow the operators to make a profit from current investments in GSM, taking advantage of the existing coverage while familiarizing their subscribers with new services provided by UMTS.
At the level of functional architecture, a Type 2 terminal comprises an electronic card responsible for performing tasks specific to GSM and an electronic card that performs tasks specific to UMTS. In this type of terminal, at a given moment, one card is necessarily active and the other is inactive.
Consequently, at that moment, only communication with the network associated with the active card (GSM or UMTS) is possible. The inactive card can only perform measurements on neighboring cells belonging to the other radio access network (GSM or UMTS). The measurements possibly serve for switching automatically from a GSM network to a UMTS network and vice versa.
Recall that in a single-mode terminal (GSM or UMTS), in order to avoid relative frequency errors between the reference clock in the electronic card (GSM or UMTS) and the clock of the base station of the network (GSM or UMTS), it is necessary for the terminal to be synchronized with the associated network in time and frequency to be able to decode and then demodulate information coming from the base station. These errors may have several causes among which are variations of temperature or even aging of components of the terminal.
In order to compensate for these errors, a very common technique used in the prior art called AFC (Automatic Frequency Control) consists in ensuring frequency synchronization with the current base station to be as precise as possible with the aid of a closed loop that depends on a reference frequency value. The loop is supplied with estimations of frequency errors measured on the downlink channels that the mobile is listening.
In a Type 2 terminal with two electronic cards, GSM and UMTS, each having its own clock, synchronization between the two cards as well as between each card and the current cell presents a problem in that the AFC specific to respective cards may never converge because one of the cards is inactive for a very long period. But, one condition for AFC to converge is exactly that the terminal be able to read regularly information on those channels. Thus, when the mobile tries to perform measurements on surrounding cells, frequency errors will be such that taking of such measurements may not be accomplished.
The object of the invention is to resolve the inconveniences of the prior art described above.